1. Field of the Invention
The present invention relates to a field emission electrode which emits electrons by field emission, a manufacturing method thereof, and an electronic device.
2. Description of the Related Art
Field-emission cold cathodes can emit electrons into a vacuum space by applying an electric field to their emitter, and have gained attention as electron emission elements to replace hot cathodes. Various researches have been made to achieve a smaller threshold field intensity (a field intensity which will cause an emission current of 1 mA/cm2) and more stability and uniformity of emission currents.
Techniques for improving the electron emission characteristic of a field emission cold cathode have roughly two tendencies.
One is to make searches into the structures of electron emission materials to achieve a structure for a more enhanced electric field concentration. By sharpening the tip of an electron emission material from which electrons are to be emitted, a strong electric field that can pull out electrons is formed near the tip, which allows electrons to be emitted at a low applied voltage. Hence, many reports about applications of carbon nano tube (hereinafter referred to as CNT), carbon nano fiber, etc. as field-emission type electron emission elements have been made so far. Carbon nano tube is a carbon material with sharp tips that has a minute structure of a nanometer size with a high aspect ratio.
Unexamined Japanese Patent Application KOKAI Publication No. 2003-59391 discloses a manufacturing method of a field emission cold cathode using CNT.
According to this manufacturing method, a substrate to serve as the electron emission electrode is etched to be provided with bosses and recesses thereon, and the surface of the bosses is covered with a conductive material such as Al and then has CNT adhered thereon. After grains of the CNT, which has been produced aside from the substrate by arc discharge, are adhered to the bosses of the substrate by electrophoresis, the conductive material is melted to flow into clearances of the CNT.
The other tendency for improving the electron emission characteristic of the field emission cold cathode is to reduce the potential barrier near the surface of the electron emission material, which is the site to emit electrons therefrom.
It is effective for this purpose to use a material having a small electric affinity as the electron emission material. Especially, diamond has not only negative electron affinity but also a high degree of hardness, and thus is chemically stable. Diamond is therefore suitable as the material for electron emission elements.
However, in case of an electron emission element made of diamond, the higher the crystallinity of diamond is, the lower the basic electric conductivity is, giving rise to a problem that a favorable electric contact is hard to obtain between the diamond and the substrate serving also as an electrode.
To deal with this problem, Unexamined Japanese Patent Application KOKAI Publication No. H9-161655 teaches incorporating impurities such as nitrogen in diamond thereby to improve the electron emission characteristic.
The structure for enhancing the field concentration can have a higher field concentration as the shape of the tip as the electron emission site is sharper, but becomes so less durable. The technique of Unexamined Japanese Patent Application KOKAI Publication No. 2003-59391 has to undergo complicated manufacturing steps after production of the CNT, and suffers a problem that the ratio of CNT that adheres with respect to the yield is low because the CNT grains are physically moved in the dispersion liquid by electrophoresis.
On the other hand, the electron emission element made of diamond is highly durable because of its rigid crystalline structure and is less liable to deteriorate. Furthermore, diamond has a low work function and thus can emit electrons with a low field concentration.
However, the high electric resistivity of diamond is an obstacle against meeting a condition of a field intensity of 1V/μm or less at a current density of 1 mA/cm2, which is one standard in promoting practical use of electron emission elements. This obstacle cannot have been counteracted so far by any of the enhancement of field concentration by improving the surface structure of the emitter film, impartment of a lower resistivity to diamond by doping impurities, improvement of the electric contact between diamond and the conductive substrate.
Accordingly, an object of the present invention is to provide a field emission electrode easy to manufacture and having a high current density at a low field intensity, a manufacturing method of such an electrode, and an electronic device.